From what I've been able to find out, online sources recommend around 205ºC for PLA and around 240ºC for ABS. But these are only guidelines, of course. Optimal printing temperature can be different depending on the printer, the filament, the model and other slicer settings.
For example, I've had success printing black PLA at 190ºC, but silver PLA of the same brand is giving me trouble. I'm having a hard time figuring out the general rules. So I would like to see a general guide for this, based on (at least) the following questions:
Which known factors before a print can help determine the right extrusion temperature? Obvious example: ABS vs PLA
What can happen during or after a print when the temperature is too low?
What can happen during or after a print when the temperature is too high?
An answer to the first question could take the form of a lookup table, or similar. The second and third could help someone adjust their temperature based on the symptoms of a failed print.
I understand that the failure or success of a print can depend on many more factors than extrusion temperature, but I didn't want to make this question too general. I may later ask the same question for other settings (e.g., print speed). However, do let me know if this question should be expanded or improved to make it more useful.
Some manufacturers give a recommended temperature which would be the best place to start at and adjust from there. Otherwise you could print calibration objects and find the best temp that way. Here is a temp tower for that purpose. Mostly it is a trial and error process with all the different printers out there. What might work on one printer may not work on another.
The object may come apart if the temperature is too low as the plastic won't bond well between layers. You may also get a rough surface on the object and the filament may also jam in the hotend as well.
Normally temperatures that are too high, your bridging wont be as good and you will get a lot of stringing. Stringing example, more filament will ooze out before retracting if your temperature is too high. Your overhangs may also curl at the edges as well.